Orbital is right, you can do it while its open anyways. Exactly like this (assuming you have W7 and since you have a 5850 i suppose you do) Local Disk > Program Files (x86) > MSI Afterburner > MSIAfterburner.cfg
After that right click on it, Properties > Security > Edit > (Your now in the editing of permissions of that cfg file)
Click on your user, whether you be the admin or the users it'll be listed as
SYSTEM
Administrators (Name\Administrators)
Users (Name\Users)
Click on which ever applies to you, then check the Allow Full Control. Apply then your done.
HOWEVER! Note you must be Admin to do this.
EDIT: After you finished with the permissions stuff, open the cfg with notepad and change the EnableUnofficialOverclocking = 0 to EnableUnofficialOverclocking = 1
That EnableUnofficialOverclocking is under ATIADLHAL in the MSIAfterburner.cfg